Field
The present disclosure relates generally to wireless devices, and more particularly, to range estimations between wireless devices that involve adaptive triggering of round trip time (RTT) ranging.
Background
Time of flight measurements may be used to estimate the distance between a transmitter and a receiver. In this approach, the transmitter may measure the time of departure of a first packet, and the receiver may measure the time of arrival of the first packet. The transmitter may then send the time of departure to the receiver in a second packet. The receiver may then compute the time of flight of the first packet by taking the difference between the time of arrival (measured locally from the first packet by the receiver) and the time of departure (obtained from the transmitter with the second packet). This is the known as the Fine Timing Measurement—Acknowledgement (FTM-ACK) or the RTT unicast mechanism. In this scheme, the clocks on the transmitter and the receiver may be assumed to be not synchronized.
RTT ranging may be accurate and may be possible to achieve centimeter-level accuracy. However, the scalability of the prior RTT ranging schemes may be questionable in cases of multiple devices. As an example, in the vehicular dedicated short-range communication (DSRC) standard, the number of message exchanges scales as the square of the number of devices, and in cases where there are around 100 vehicles in the radio range, the number of message exchanges required would be high and imposes significant burden on the computing elements of the vehicles.
In addition, coarse ranging estimates may be obtained using broadcast messaging on a channel. For example, DSRC message timing information may provide a certain granularity of positioning accuracy. Similarly, range estimates may be obtained on Long-Term Evolution Direct or LTE-D spectrum, since the clocks of the transmitter and receiver may be synchronized to a certain level of accuracy. However, in cases where messages are lost or the estimation accuracy drops, such coarse ranging estimates may not suffice.